1. Field of the Invention
The present invention relates to a measuring system and a method for measuring a position of a target (a point on an object to be measured) by receiving light on a light-receiving surface of a light-receiving device mounted to a robot.
2. Description of the Related Art
A touchup method is known as a method for determining a position of an object near a robot and, in particular, a position of a target on the object to be measured. This method includes steps of: predetermining a position of a tool center point (TCP) with respect to a coordinate system fixed to an end of an arm of the robot (mechanical interface coordinate system), moving the robot in manual mode such as jog feed so as to accurately coincide TCP with the target (point to be measured), getting position data of the robot so as to determine the position of the target.
Another method is also known in which a robot and a camera are combined. The camera is attached to the end of the arm of the robot. The camera is moved by the robot for stereo view measurement, whereby the position of the target relative to the robot is found. As is known, this method requires a calibration in which a sensor coordinate system generally fixed to a body of the camera is determined and, a position of the sensor coordinate system and a position on an image of the camera Corresponding to the position of the sensor coordinate system are calculated. A detailed explanation of this calibration is omitted since the calibration is well-known in the art.
Further, in the method in which the robot and the camera are combined, a positional relation between the robot and the camera must be calculated previously in addition to the calibration. This calculation is often referred as a combination of the robot coordinate system and the sensor coordinate system. Examples of this are disclosed in following documents: Roger Y. Tsai and Reimar K. Lenz, “A New Technique for Fully Autonomous and Efficient 3D Robotics Hard/Eye Calibration”, IEEE Trans. on Robotics and Automation, Vol. 5, No. 3, 1989, pp. 345-358, and Japanese Patent Application Unexamined Publication No. 10-63317.
If both of the calibration and the combination of the coordinate systems has already completed, a visual line directed from the camera to the target can be calculated. Therefore, by moving the camera to two positions and measuring the target, a three-dimensional position of the target relative to the robot can be calculated as an intersection point of two visual lines.
However, the above two methods (i.e., the touchup method and the combination method or the camera and the robot) have problems.
First, in the touchup method using the robot, it is difficult to measure with high precision because of a possible setting error of TCP relative the robot (or the mechanical interface) and a positioning error of TCP to the target during the touchup operation. In both of setting and positioning of TCP, an operator must move the robot by jog feed and coincide TCP of the robot with a desired position. In this case, the setting and the positioning have different precision levels depending on the orientation of the robot when setting and positioning are carried out or depending on operator's skill. Particularly, because positioning is carried out based on visual measurement, even a skilled operator cannot achieve high-precision work. Further, as a TCP is approached or contacted to the target, interference (or damage of TCP and/or the object to be measured) may occur.
On the other hand, the method in which the camera is attached to the end of the arm of the robot and is moved by the robot for stereo view measurement, is based on measurement of the target by the camera. Therefore, the precision level of the method is stable by eliminating a factor including a human error such as visual inspection and the security of the method is high because the method uses non-contact type measurement. However, as described above, the combination of the coordinate systems in the calibration is required and, the effort of operation required for the calibration and its preparations is by no means small. The reason for this will be briefly explained with referring to FIG. 11.
FIG. 11 shows a typically arrangement used for the calibration of prior art. A robot denoted by numeral 1 is controlled by a robot control device 5. A camera 4 is attached to around the end of the arm of the robot 1. The camera 4 is connected to an image processing unit 2 including a monitor 3 having a LCD or a CRT. Numeral 6 denotes a fixture shaped like a plate prepared for the calibration or a calibration plate. For example, the calibration plate 6 has a dot pattern including a known array of dots. The image of the calibration plate is taken by the camera 4 and is analyzed by the image processing unit 2. Then, the sensor coordinate system is determined and, parameters indicating the position of the sensor coordinate system and the position on the image of the camera corresponding to the position of the coordinate system are calculated as calibration data and are stored.
Next, the coordinate systems regarding the robot and the camera are combined based on the conventional technique such as described above. Concretely, using the example of FIG. 11, matrix data representing relative positional relation between the sensor coordinate system Σs and the mechanical interface coordinate Σf of robot 1 are obtained. Such a series of operations requires complicated preparations and the exclusive calibration plate 6. Further, in general, a light receiving part (for example, CCD array) or an imaging lens of the camera may have a geometric distortion. In particular, a lens often has a significant distortion. The distortion of the lens is larger at the perimeter of the lens, therefore, calculation of the visual line from a focal point of the lens to the target may include a small or a large error, depending on the position of the target on the image.